Introduction

Material fatigue is the most common form of material failure in metal structures. The phenomenon is characterized by cumulative material damage due to repeated loading and may occur at stress levels far below yield stress.

The picture to the right shows a bicycle pedal arm that has undergone a complete failure due to fatigue. Here the damage was initiated at the bottom of the failure surface, and progressed upwards leaving a characteristic "wavy" pattern behind as it worked its way through the material of the increasingly weakened structural element. Eventually, the pedal arm became impaired to the point where it could no longer sustain the applied load, and therefore ruptured in what appears to have been a brittle failure.

Metal fatigue progress in three subsequent main stages:

initiation
crack growth
final failure

The fatigue life of initially crack-free parts is commonly dominated by the initiation period. During this stage, the damage is small and the mechanical strength is approximately unperturbed. Sufficiently harsh conditions will take the advancement into the crack growth phase. Here, present micro-cracks break through the boundaries between the grains of the material and become macro-cracks. Final failure occurs if the macro-cracks grow large enough to eliminate the component's capacity to sustain applied loads.

Welded joints are particularly susceptible to fatigue. This is due to several facts, and one of them is the that welds have significant initial cracks. The discussion of fatigue in welds is dedicated a major separate section.